Cause of Cold Conditions in Early Winter But Warm Anomalies in Late Winter 2014/15 over East Asia

Subseasonal surface air temperature variability reversals occurred in the early and late winter of 2014/15 over East Asia, with warmer anomalies dominating in January and February but colder conditions prevailing in December.

In investigating this topic, a research team across Institute of Atmospheric Physics at Chinese Academy of Sciences, Nanjing University of Information Science and Technology, the Norwegian Institute for Air Research, the Geophysical Institute, University of Bergen, and Nanjing University of Information Science and Technology found that reduced autumn Arctic sea-ice cover (ASIC) and warm sea surface temperature (SST) anomalies in the Ni?o4 region under the positive Pacific Decadal Oscillation (PDO|+) phase that occurred in winter 2014/15 provide a plausible explanation for the colder-than-normal December and warmer-than-normal January–February of this year.

Specifically, under PDO|+ phase modulation, warm Ni?o4 SST anomalies are associated with a subseasonal delay in tropical surface heating and subsequent Hadley cell and Ferrel cell intensification in January–February, linking the tropical and midlatitude regions. Consistently, the East Asian jet stream (EAJS) is significantly decelerated in January–February and hence promotes warm anomalies over East Asia. The reduction in ASIC under PDO|+ is related to cold SST anomalies in the western North Pacific, which increase the meridional temperature gradient and generate an accelerated and westward-shifted EAJS in December. The westward extension of the EAJS is responsible for eastward-propagating Rossby waves triggered by the declining ASIC and thereby favors the connection between ASIC and cold conditions over East Asia.